The present invention relates to the structure of an antenna for a portable radio communication apparatus and capable of achieving a desirable characteristic in both of the extended position and retracted position of a whip antenna thereof.
An antenna for use in a portable radio communication apparatus should preferably be made up of a whip antenna having an electric length of .lambda./2 (1/2 wavelength) and a helical antenna or similar miniature antenna. While conversation is held on the radio communication apparatus, the whip antenna is used in order to reduce body deterioration. In the stand-by condition of the radio communication apparatus, the helical antenna is used so as not to degrade portability. It has been customary with a portable radio communication apparatus to combine a retractable whip antenna and a helical antenna such that the whip antenna operates when it is extended or the helical antenna operates when the whip antenna is retracted.
A decrease in the size of a portable radio communication apparatus has made it difficult to provide a whip antenna with an electric length of .lambda./2. A helical antenna may be coaxially connected to the tip of a whip antenna in order to implement the electric length of.lambda./2 (1/2 wavelength) and reduce body deterioration. For example, Japanese Patent Laid-Open Publication Nos. 5-343907 and 6-216630 teach a variable length whip antenna and a retractable whip antenna, respectively. Further, Japanese Patent Laid-Open Publication No. 7-273524 proposes an antenna having a fixed helical antenna having a 1/4 wavelength.
In each of the conventional antenna structures, the whip antenna and helical antenna are connected to each other. This brings about a problem that when the whip antenna is retracted, the resonance frequency of the helical antenna and corresponding impedance matching condition cannot be optimally adjusted due to the influence of the whip antenna. To conventionally solve this problem, adjustment is made in consideration of balance between the extended position and the retracted position of the whip antenna. This kind of adjustment, however, prevents the whip antenna and helical antenna from fully utilizing their individual characteristics. Moreover, the power transfer efficiency is lowered due to an increase in input impedance. In addition, the adjustment of the helical antenna taking account of the above noted balance prevents the helical antenna to be optimally adjusted in the retracted position of the whip antenna, degrading the radiation efficiency in the retracted position.
A problem with the variable length whip antenna is that in the retracted position not all power is fed to the helical antenna because the feed relies on capacitive coupling, lowering the radiation efficiency in the retracted position.
In all of the conventional variable length whip antenna, retractable whip antenna and antenna with a fixed 1/4 helical antenna, not all power output from the helical antenna, which plays the role of a matching circuit between the whip antenna and a radio circuit, can be transferred to the whip antenna. As a result, the helical antenna adjoining a feed portion operates as an antenna and radiates. This prevents the whip antenna from achieving an expected radiation efficiency and therefore an expected optimized characteristic alone.